1. Field of the Invention
The present invention relates generally to an apparatus and methods for orienting tubulars in wellbores. More specifically, the invention relates to an apparatus and method for rotationally orienting an opening or window in a casing or tubular string in a non-vertical wellbore. More specifically still, the invention relates to an apparatus and methods whereby the shape of the apparatus, as well as the relationship between the center of gravity and the geometric center of the apparatus, is used to rotationally orient the casing or tubular string in a non-vertical wellbore.
2. Description of the Related Art
Lateral wellbores are routinely used to more effectively and efficiently access hydrocarbon-bearing formations. They are typically formed from a central wellbore. In one conventional method, a window is formed in casing after the casing is located in the central wellbore. In some instances, the window is formed in the wellbore with a milling tool prior to the formation of the lateral wellbore. In other instances, the casings inserted into the central wellbores contain pre-milled windows to allow the lateral wellbore to be formed without the prior steps of forming a casing window. Because lateral wellbores xe2x80x9ckicked offxe2x80x9d from central wellbores are so popular, they are sometimes formed from central wellbores that are themselves non-vertical and are in some cases horizontal. When utilizing a pre-milled window, it is necessary to provide a means of ensuring the section of the casing containing the pre-milled window is in the desired rotational orientation after being axially positioned in the central wellbore. Rotational orientation ensures that the lateral wellbore will be directed towards the desired formation.
A conventional method of ensuring the correct rotational orientation of the casing is to use a survey tool, which is well known in the art, to detect the actual window orientation. Once the actual orientation is known, the entire casing is rotated from the surface of the drilling rig, until the survey tool detects the window is in the desired orientation.
The casing string above the window may be several thousand feet long, and therefore rotation of the entire casing places significant torsional stresses on the casing. The survey tool is typically run into the well on a wireline in a separate run. The equipment is expensive, not always accurate and its use requires valuable rig time. The inherent weakening of the casing in the section where the pre-milled window is located further aggravates the problems associated with high torsional stresses. The combination of high torsional stresses and weakness in the casing near the window can lead to failures of the casing, resulting in significant delays and additional expense.
An alternative method of ensuring the correct rotational orientation of a casing window utilizes an apparatus that de-couples a lower section of the casing from an upper section when the casing is placed in tension. The apparatus and method which allow the independent rotational movement of the two sections of casing are disclosed in U.S. Pat. No. 6,199,635, issued on Mar. 13, 2001 to the inventor of the present invention. That patent is incorporated by reference herein in its entirety. In this method, a survey tool is used to detect the rotational orientation of the casing window. The casing is then placed in tension by using a drill string to lift up on the casing at the surface, thereby de-coupling a section of the casing (including the section with the pre-milled window) downhole of the device from the remaining portion of the casing. A drill string can then be used to rotate the section of the casing containing the pre-milled window independent of the upper portion of the casing. Because a pre-milled window is usually near the end of the casing, this method has the advantage of eliminating the need to rotate a majority of the casing, thereby reducing torsional stresses on the casing and the chance for a casing failure. However, this method requires the use of a survey tool and a separate run into the well, thereby increasing the time and costs.
When installing casing in a non-vertical wellbore, it is also necessary to provide a means for offsetting the natural tendency of the casing to rest against the bottom or xe2x80x9clow sidexe2x80x9d of the wellbore. This is needed to ensure that cement, which fills the annular area between the outside of the casing and the wellbore, completely surrounds the circumference of the casing and provides a good bond between the casing and the walls of the wellbore.
This need is typically met through the use of centralizers, which are devices placed around the outside of the casing. These devices support the casing in the center of the wellbore so that it is not resting on the bottom of the non-vertical wellbore. Conventional centralizers do not, however, impart any rotational forces on the casing.
When installing casing with a pre-milled window in a wellbore, it is further necessary to provide a means of temporarily covering the pre-milled window in the casing in order to allow cement to be pumped through the end of the casing and into the annular area between the casing and the wellbore.
The need to cover the window is typically met through the use of a temporary inner liner within the casing. The inner liner does not contain a window (as the casing does), and therefore allows cement to be pumped through the section of casing having the window and into the annular area between the casing and the wellbore. After the cement has been pumped through the inner liner, the liner is removed or destroyed by drilling and the window in the casing is exposed. The inner liner is typically fiberglass or a similar drillable material and does not provide any increased structural rigidity to the weakened section of the casing containing the pre-milled window during the casing installation process.
Typically, casing is run with a float shoe at a lower end thereof. The float shoe facilitates cementing and prevents the backflow of cement into the casing or tubular string. This is accomplished through the use of a check valve incorporated into the float shoe. Conventional float shoes, like centralizers, do not impart any rotational forces on the casing.
There is a need therefore, for an apparatus and method to rotationally orient a tubular string in a non-vertical wellbore that will overcome the shortcomings of the prior art devices and methods. There is a further need for an apparatus and method to rotationally orient a tubular string having a pre-milled window in a non-vertical wellbore without placing significant torsional stresses on the tubular string in the area of the window. There is still a further need for an apparatus and method to rotationally orient a tubular string in a non-vertical wellbore without the expense of survey tools or extra additional trips into the well.
There is a further need for an apparatus and method which will both centralize a casing or tubular string within a non-vertical wellbore and impart rotational forces to the casing or tubular string so that it may be placed in a desired rotational orientation.
There is yet a further need for an apparatus and method which will both temporarily cover a pre-milled window in a casing and provide increased structural rigidity to the weakened section of the casing containing the pre-milled window during the casing installation process.
There is a further need for an apparatus and method which will temporarily cover a pre-milled window in a casing, and serve as a pressure barrier to contain any cement which is pumped through the casing section containing the pre-milled window during the casing installation process.
There is yet a further need for an apparatus and method which will provide increased structural rigidity to the weakened section of the casing containing the pre-milled window during the casing installation process.
There is a further need for an apparatus and method which will both prevent the back flow of cement into the tubular string or casing and will impart rotational forces to the tubular string or casing so that it may be placed in a desired rotational orientation.
The present invention relates generally to an apparatus and method for orienting tubular strings in wellbores. One embodiment of the invention utilizes the inherent eccentricity of a non-vertical wellbore to provide a means of orienting a portion of casing that contains a pre-milled window.
Any device such as a float shoe, outer sleeve, or centralizer that is mechanically attached to the casing near a pre-milled window may incorporate the present invention. The device is manufactured to include an eccentric portion that generally matches the cross-sectional profile of directional wellbore. Either or both the conforming shape and the gravitational effects on the eccentric portion combine to rotationally orient the device and casing to the wellbore.